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Rpm question at idel?

Old 12-11-2014, 05:08 AM
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Default Rpm question at idel?

1986 Wellcraft Nova 2
Twin (stock 260 mercruisers alpha 1's) with thru hulls


This just hit me today while staring at my boat wishing spring time would hurry up. But last yr. When I bought my boat (May) before I went out each time I would always let her idel a few minutes warm up etc etc. And when she first fired and got warm to put in gear and ease out of the no wake zone it would always drop down to around 650-750 rpm range when first engaged into gear. But after a couple minutes of ideling it would bump up to almost a 1,000 rpms out of no were. But if I would hit the drive trims she would idel back down for a short minute then back up. Is it the alternators starting to charge to make it kick up in rpms? Each engine has its own battery (which are new) just seems like once in gear and or its warmed up it should idel down? The boat came from Michigan. Now.I have her in southern Tennessee. Anything to do with altitude change? Note: carbs are stock quadrajets 4bbl's any answer to this would be great thank you!
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Old 12-11-2014, 05:14 AM
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And most days in the summer here were I like are 85+ degrees with high humidity if that's a factor?
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Old 12-11-2014, 08:06 AM
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Sounds like a carb/choke issue. Alternators don't generally make an engine idle high especially on a non computerized engine. Potentially timing?
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Old 12-11-2014, 08:18 AM
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Originally Posted by I.C.U.Lookin
Sounds like a carb/choke issue. Alternators don't generally make an engine idle high especially on a non computerized engine. Potentially timing?


Could it be carb and choke if both do it at the same time jt seems? Or one will do it then the other will do it later. But when I hit both drive trims up for a minute and then back down they will idel back down its like when it goes under a load running trim pumps they idel down?
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Old 12-11-2014, 10:00 AM
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It actually seems like you have a few things going on here. The reason your idle will go down when trim is because of a low voltage/ current issue. The pumps draw a lot of curent and can actually cause the idle to go down because of a weak battery or alternator.

As for the idle going higher when the engine warms up, this can actually be a vaccum leak. There could actually be a leak under the carb, along the top side of the intake or even drawing air from the crankcase. I've seen the latter on some drag engines.

Maybe ask the old owner if the intakes were ever pulled??

Hope this helps
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Old 12-11-2014, 11:04 AM
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The thunder bolt iv ignition seems add a few degrees of timing right off idle. Meaning if the idle wanders up just a bit, timing is added which makes it go up higher. since it is not a centrifugal weight advancing the timing it is not a smooth increase, it is electronic advance and more of a "step" in timing all at once. Not sure if this makes sense but I noticed it on my small block.
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